[skido]

Well, I have tried that as well. I disabled my Norton firewall but it did not affect to the problem.

Then anti-spam programs (Windows Defender, Ad-Aware) are only active while scanning is in progress.

What comes to my Anti-Virus sw it has no setting it can be disabled temporarily and I have thought about that as one problem source. But I have gone through all configuration data of anti-virus sw and haven't found anything suspicious which might affect. In addition, anti-virus sw usually informs clearly if it decides to take some action.